If a character with 20 Defense gets attacked by 20 or less Attack, will there be zero damage?
If a character with 20 Defense gets attacked by 21 Attack, will there be instant death or 1 damage?
With physical damage(atk/def), damage scales up the more attack you have compared to the enemy's defence in a LINEAR fashion
With magic damage, damage is 0 when enemy spell defence > your Spell attack(but elemental damage is different and is only affected by elemental defence) and the damage scales very quickly when you have more spell attack.
Ex: 40k SA vs 45k sd = 0 dmg
47k SA vs 45k SD = 20k dmg
60k SA vs 45k SD = 100k dmg
This is a rough estimate and not factoring critical hits as it is calculated differently and i'm not quite sure what the formula for it is.